The boxy shape of conventional tractor-trailer combinations is dictated by a need to provide a large cargo volume within the maximum allowable dimensions that are fixed by law. In the past, the low aerodynamic efficiency of these vehicles was considered of little importance. However, the high cost and uncertain availability of fossil fuels has increasingly focused attention on reducing the aerodynamic drag that accounts for more than half of the fuel consumption of large trucks in long-haul highway operations on fairly level pavement at high speed. These conditions correspond to normal Interstate Highway Conditions.
The potential economic benefit of drag-reduction apparatus can be evaluated based on the fuel that would be saved using the apparatus, the saving being independent of vehicle weight load and rolling resistance. For example, a conventional tractor-trailer combination weighing about 30 to 80,000 pounds and having a cross-sectional area of 100 square feet and a drag coefficient of 0.55 typically consumes about 16 gallons of diesel fuel per hundred miles at 60 MPH, on nearly level ground. The fuel costs $40 at a price of $2.50 per gallon. The truck requires about 165 horsepower to overcome about 150 pounds of rolling friction and internal loading and about 975 pounds of aerodynamic drag. A ten percent reduction in the aerodynamic drag results in a savings of (97.5/550)*60*(88/60)=15.6 HP. This is equivalent to a savings of about 1.83 gallons or $4.57 per hundred miles, independent of vehicle loading. On a trip of 2,000 miles, the savings is more than $91.50.
At higher speeds, air drag is exponentially greater and there is a corresponding greater reduction in drag for even greater savings. For example, on a 2,000 mile trip at an increased speed of 70 MPH, the resulting aerodynamic drag increases 45% and a ten percent reduction results in a savings of about $128 in fuel costs.
Further, when the effects of wind are considered, the potential savings are even greater. Moreover, the cost and availability of diesel fuel are subject to change, potentially making aerodynamic drag a critical factor in shipping economy.
Lastly, the airfoil shape, although designed for fuel economy, and foreshortened to comply with existing regulations for vehicles on the National Network and the highways of the several states, will nonetheless provide an important increase in the dynamic stability of those vehicles upon which it is installed, while posing no increased hazard to other vehicles or their passengers.
Although there are a variety of devices in the prior art for improving the aerodynamics of truck vehicles, these devices exhibit a variety of disadvantages. For example, many simply are ineffective because they provide little or no actual drag reduction in use. Some are awkward to use because they interfere with normal loading and maintenance operations. Others are heavy and bulky, being difficult to install, remove, and store when not in use. Many are unsafe because they tend to interfere with lighting visibility and most are costly to produce and install. Most of these devices are not in compliance with existing state laws which regulate maximum width and length of vehicles. Most of these devices do not satisfy the conditions of the federal regulations which mandate exclusions to said state laws for the purpose of facilitating aerodynamic enhancement of bluff body vehicles.
Thus, there remains a need for alternative devices and systems for enhancing the aerodynamic performance of a vehicle.